Use SkSafeMath for text blob storage calculations

Change-Id: I028895dee81d99b1fa2a9acfa3db3f4bcb0f8f64
Reviewed-on: https://skia-review.googlesource.com/73823
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/src/core/SkSafeMath.h b/src/core/SkSafeMath.h
index f781c2c..1005d06 100644
--- a/src/core/SkSafeMath.h
+++ b/src/core/SkSafeMath.h
@@ -43,6 +43,11 @@
         return a + b;
     }
 
+    size_t alignUp(size_t x, size_t alignment) {
+        SkASSERT(alignment && !(alignment & (alignment - 1)));
+        return add(x, alignment - 1) & ~(alignment - 1);
+    }
+
 private:
     uint32_t mul32(uint32_t x, uint32_t y) {
         uint64_t bx = x;